1. Field of the Invention
The present invention is directed to a compact disc (CD) system of the optical or optomagnetic type capable of reading discs recorded in the standard CD-Audio and CD-Read Only Memory (CD-ROM) formats, reading and writing discs in the CD-recordable (CD-R) format and/or the newly proposed CD-erasable (CD-E) format, as well as reading/writing in a direct access storage device (DASD) format, and, more particularly, to a system that uses the typical components of a CD-Audio/ROM system and low cost additional components to write/read data on a disc in both the CD-Audio/ROM and CD-DASD formats.
2. Description of the Related Art
The Compact Disc.TM. (CD) optical data storage system was originally designed as a consumer product that would read (playback) digitized audio information in a sequential fashion, much like a tape, from unprotected plastic discs that would be extensively handled. Accordingly, the recording format (i.e., the precise manner in which the data stored on the disc is mapped to the trail of physical marks written on the disc's surface) of this system is optimized for the continuous retrieval of data from the disc and also to mitigate the affects of relatively large defects (such as scratches and fingerprints) on the reliability of the data recovered from the disc. The CD-Audio recording format therefore handles (during reads & writes) input and output data (i.e., user digital audio data) in small, contiguous 24-byte blocks called "frames" and further causes the data that comprises a single frame to be widely distributed on the surface of the disc when it is recorded. Moreover, there is no provision in the recording format for the precise addressing of an individual frame (i.e., allowing the CD playback device to determine the exact physical location of any of the constituent bytes in a frame on the disc). In fact, the only means of locating information on the disc is via the information carried by a separate control & display (C&D) channel that is multiplexed with the main (digital audio) data channel.
The specific item of information carried by the C&D channel that provides the vehicle for locating information on the disc is the "absolute-time-on-disc" which is the elapsed disc playing time relative to the beginning of the recorded disc information area. Absolute time information is updated with a granularity of 1/75th of a second. Since exactly ninety-eight 24-byte frames of audio data are played each 1/75th second, the C&D channel can be used to "segment" the contiguous audio data stream channel on the disc into data blocks that contain 98.times.24=2352 bytes. A main (audio) data channel block that consists of 98 contiguous frames, or 2352 bytes of digital audio data, is called a "C&D Section". However, a given 2352-byte C&D Section cannot be precisely located on a disc; this is due to the fact that the CD-Audio disc recording standards provide for a tolerance of .+-.1 second between the start of the C&D channel's absolute-time-on-disc information and the start of audio program data on the disc. (Note: The absolute time value is specified to be 0 minutes, 0 seconds and 0 seventy-fifth seconds at the start of the first data (audio) track of the disc, which immediately follows the disc's lead-in track. The lead in track is the first track in the disc's information area: absolute time increases from some negative value during the lead-in track such that it becomes zero exactly at the end of the lead-in track)
In 1984, or thereabouts, a new version of the CD system known as Compact Disc Read-Only-Memory (CD-ROM) was introduced. CD-ROM was designed as a playback-only computer peripheral and CD-ROM drives connected to a computer could be used to retrieve files of data from a prerecorded disc in response to commands from a requesting application program. To control the cost of the CD-ROM drives and to provide them with the capability to "play" CD-Audio discs, the recording format of the CD-Audio system was fully retained in the CD-ROM system. This enables CD-ROM discs, which each may hold over 600 Mbytes of data, to be produced on the same manufacturing line as CD-Audio discs and allows CD-ROM drives to share components with CD-Audio players. The CD-ROM system has proven to be a commercially successful, low cost means of distributing very large data sets and application programs to computer users.
Computer operating systems (i.e., the programs that, among other things, manage the storage and retrieval of data needed by application programs that are running on a computer) are designed to move data between the central processing unit (CPU) and the computer's storage peripherals in units, or blocks, called "data clusters". Clusters always contain 2.sup.n bytes, where n is an integer (usually n.gtoreq.10). Computer peripherals, such as hard disk drives, therefore, are designed to handle data in blocks called sectors that each contain 2.sup.m bytes of arbitrary-valued data that could be assigned to a specific cluster that belongs to some user data file (usually m is an integer .gtoreq.8). Because of the way that information on a compact disc is segmented by the timing information in the C&D channel, the CD-ROM system employs sectors that contain 2352 bytes and, in the most widely used embodiment of CD-ROM, each sector holds 2048 "user bytes", or arbitrarily valued bytes that could belong to a user data file.
The 2352 byte CD-ROM sectors are logically defined by exactly mapping them. i.e., assigning their contents to, 98 contiguous 24-byte frames. However, as was mentioned previously, the data in each of these frames is widely distributed along the disc's spiral data track. In fact, data stored on the disc data track is organized as contiguous 33-byte blocks called "eight-to-fourteen modulation (EFM) frames." Each EFM-frame contains one byte of (multiplexed) C&D channel information, eight bytes of error correction code (ECC) parity data and 24-bytes of user data. Each byte of user data in a given EFM-frame is obtained from twenty-four different 24-byte data frames that are distributed over 106 contiguous data frames. Thus, the 24 bytes of a given data frame are distributed over 106 consecutively recorded EFM-frames on the disc. But, in order to recover the 24 bytes of a single data frame from the disc, 111 consecutive EFM-frames have to be retrieved (the additional 5 EFM-frames contain all the ECC parity data needed to complete, and thereby render decodable, the ECC codewords that protect the specific 24-byte data frame).
Recall that the C&D channel's absolute-time-on-disc information segments the main data channel on the disc into 2352-byte C&D Sections (this is true for CD-ROM discs as well as CD-Audio discs because their low-level recording formats are exactly the same). Unfortunately, this segmentation cannot be used to precisely define where (on a CD-ROM disc) the boundaries, or start, of a given sector resides. This is due to the fact that the control & display (C&D) and main data channels are not aligned (as noted previously). Thus, since a sector may start in (that is, the first byte of the recorded sector may occur in) any arbitrary 33-byte EFM-frame on the disc, the "offset" between the boundaries of CD-ROM sectors and the C&D Sections on the disc will be &lt;.+-.98 EFM-frames (or equivalently, &lt;.+-.1/75 second since EFM-frames are synchronous with data-frames; one EFM-frame is formed for each data frame that is input to the CD-Audio/ROM encoder). To facilitate locating information on a CD-ROM disc each sector contains "address" data, which is used by the CD-ROM drive's controller to identify specific sectors (the computer operating system also uses a translation of this address data, together with the disc directory and file allocation tables, to identify how the user data in the sectors relates to the files on the disc). Thus, to retrieve a specific sector from a disc the CD-ROM drive must first read approximately 300 sequential 33-byte EFM-frames from the disc and then deliver the data contained in them to the drive's controller which "finds" the 98 sequential 24-byte data frames that comprise the sector and extracts the desired user data. Even if the offset between sectors and C&D Sections is zero, more than 200 contiguous EFM-frames still must be read to retrieve a single sector. This is because entire or complete error correction codewords must be recovered before decoding of the ECC words can be accomplished; the data needed to complete all of the error correction codewords that protect data that resides in the sector of interest is distributed over 208 contiguous EFM-frames. The underlying CD-Audio recording format specifies this wide scattering of the data that comprise individual codewords to enable the correction of long data error bursts that may be caused by large defects on the disc caused by handling.
In 1990, the Compact Disc-Recordable (CD-R) system was introduced. A CD-R "writer" can write digital audio data or logical CD-ROM sectored data to recordable discs that can subsequently be read in any CD-Audio player or CD-ROM drive (and in the CD-R writer as well). CD-R writers can write entire discs at once, or they can write a portion of a disc called a "session". In addition, the CD-R standards provide for the writing of small segments of data, e.g., a single CD-ROM sector, in one writing operation; this is called "packet writing". When appending any new information to a disc (i.e., when performing session or packet writing), however, a CD-R writer must always add the new information directly to the end of the already written portion of the spiral data track on the disc. Moreover, in packet writing, at least four "link sectors" (and usually seven to eight sectors, in practice) that contain useless (padding) data must be appended to the sectors of user data that one wants to record. These recording characteristics (i.e., sequential appending to the previously written portion of the data track and link sector overhead) result directly from the nature of the CD-ROM recording format and the underlying CD-Audio recording format.
High performance computer data storage peripherals, otherwise known as Direct Access Storage Devices (DASDs.), have recording formats that enable them to operate in a manner that is consistent with the way computer operating systems handle files. In particular, the recording formats used by DASDs cause all bytes that comprise a specific sector to be contiguously recorded along a continuous segment of the data track on the storage medium and further cause sectors to synchronously occur along the data track so that DASDs know the exact physical location of every sector recorded on their storage medium. Moreover, a DASD storage medium is subdivided into sectors prior to writing file data to it (this is done via a process known as "formatting"). Thus, a DASD can write, or read, a single sector as an independent unit and it can locate a sector anywhere on its storage medium, regardless of how much of, or what portion of, the medium is already written. These operational features allow fast file access (e.g., only a single sector might have to be rewritten if only a small part of a file is to be updated) and they are critical to overall data reliability (sectors that begin to experience data recovery errors, as reported by the DASD error correction sub-system, are retired and their contents rewritten to a new location on a portion of the storage medium that is known to be error-free).
The use of CD-writers to produce small numbers of discs that can be distributed to business and/or consumer computer users (who have a CD-ROM drive installed in their computers) is an important emerging application. However, the incorporation of CD-writers into personal computers and work stations is being impeded by the fact that they cannot perform DASD-like operations, i.e., the limited usefulness of a CD-writer makes it a very expensive peripheral from the perspective of a general user. One attempt at solving this problem is the Power Disc (PD) optical disc system recently introduced by Panasonic, which can read any compact disc (i.e., a disc that conforms to the standards for CD Audio/ROM discs) and which, in addition, will operate as a DASD. When operating as a DASD, the PD drive uses a proprietary recording format. Two drawbacks of the PD system is that it cannot use standard recordable CD discs when operating in the DASD mode and it cannot write compact discs that can be read on standard CD-Audio or CD-ROM players. The PD drive uses a proprietary disc and recording format when operating in DASD mode, i.e., it cannot write at all using a standard CD-R disc, nor can it write using the soon-to-be available CD-erasable, or CD-E disc.
An important problem to be solved, therefore, is to provide a CD-device that can write/read information in all standard CD recording formats and which has the additional capability of operating as a direct access storage device (DASD), and to do this using common CD components (i.e., conventional CD hardware and discs).